1. Field of the Invention
This invention relates to a light scanner and an image formation apparatus such as a laser beam printer using the light scanner and in particular to a light scanner using a light source comprising a two-dimensional array of light source elements and an image formation apparatus.
2. Description of the Related Art
To perform high-speed print or high-dot-density print on a laser beam printer, it is necessary to increase the number of scanning times per unit time. The number of repetitive scanning lines can also be increased to some extent by increasing the number of revolutions of a rotating polygon mirror or increasing the number of mirror surfaces, but there is a limit. Hitherto, it has been well known that a multiple beam scanning technique of scanning a large number of laser beams at a time is effective.
For example, a technique of separating light from a single laser light source element into a plurality of laser beams, modulating each by an output pattern signal, and scanning the laser beams over the scanning surface at the same time through a single rotating polygon mirror and an Fθ lens (JP-A-62-239119), a technique of making an array of a plurality of semiconductor laser elements that can be modulated separately, converting each emitted light into collimated light through a single collimator lens, and scanning a plurality of laser beams over the scanning surface at the same time through a single rotating polygon mirror and an Fθ lens (JP-B-60-33019), and the like are available. However, in the multiple beam scanning lens systems, an optical system comprising a two-dimensional array of elements making up the light source with an increase in the number of beams is not considered.
The size of each of light source elements 9 making up a two-dimensional array is a0 and the spacing between the light source elements 9 is c0, as shown in FIG. 2. To form an image on a scanned medium such as a photoconductor drum through a scanning lens system with main scanning direction magnification Bx and subscanning direction magnification By, the spot diameter of a spot 10 and the spacing between the spots become as shown in FIG. 3. That is, the main scanning direction spot diameter becomes a0·Bx, the subscanning direction spot diameter becomes a0·By, the main scanning direction spacing becomes c0·Bx, and the subscanning direction spacing becomes c0·By. As n light source elements of a two-dimensional array, arranged in the main scanning direction are rotated, the angle at which the subscanning direction spacing between the light source elements becomes equal is found according to expression (1).tan θ=1/n  (1)
At this time, the array becomes as shown in FIG. 4; when the light source element spacing is c0, the main scanning direction spacing becomes x1, x2 and the subscanning direction spacing becomes y1, y2. At this time, expressions (4) to (7) are satisfied.x1=c0·cos θ  (4)y1=c0·sin θ  (5)x2=c0 ·sin θ  (6)y2=c0 ·cos θ  (7)
The situation on scanned medium at this time becomes as in FIG. 5; the spot diameter of the spot 10 becomes a0·Bx in the main scanning direction and a0·By in the subscanning direction. The spot spacing is indicated by X1, X2, Y1, Y2. At this time, expressions (8) to (11) are satisfied.X1=Bx·x1=Bx·c0·cos θ  (8)Y1=By·y1=By·c0·sin θ  (9)X2=Bx·x2=Bx·c0·sin θ  (10)Y2=By·y2=By·c0·cos θ  (11)
Here, Y1 indicates scanning lines spacing don the scanned medium and Y1 may be equal to d. For example, to implement a 600-dpi scanning lens system using a light source of a two-dimensional square array with a0=2.1 μm, c0=90 μm, and n=2, in scanning lens system in related art, the main scanning direction magnification Bx of optical system is set to 20 times and the subscanning direction magnification By is set to 30 times, for example, to set the spot diameter to 42 μm in the main scanning direction corresponding to 600-dpi scanning and 63 μm in the subscanning direction.
At this time, θ becomes equal to 26.57 degrees from expression (1) and the scanning line spacing d becomes 1208 μm from expression (9). In contrast, if the subscanning direction magnification to set d=42.3 μm is found from expression (9), By=1.0508. However, at this time, the subscanning direction spot diameter is 2.2 μm, which is too smaller than the scanning line spacing 42.3 μm.
As described above, if an attempt is made to perform scanning corresponding to 600 dpi with the light source of a two-dimensional square array with a0=2.1 μm, c0=90 μm, and n=2, the scanning line spacing being 42.3 μm and the subscanning direction spot diameter being 42.3 μm or more cannot go hand in hand and thus a problem arises.